This invention relates to a precision heating apparatus which can locally raise the temperature of objects, such as glass cane or optical fiber, to a temperature of up to 2000xc2x0 C. and which has relative temperature stability of better than 0.1%. More specifically, this heating apparatus includes a laser and utilizes thermal radiation emitted from the laser-heated object to provide feedback that controls the laser power.
In order to manufacture photonic components such as fused fiber couplers, and long-period gratings high temperature processing such as tapering and diffusion, for example, is required to process optical materials of these components. The selection of heat source has by far the most significant impact on these processes. For high delta (HD) germanium containing fibers, time the fiber temperature has to be raised to about 1800xc2x0 C. The attributes of commonly used heat sources are compared in Table I.
Traditional heat sources have been furnace and burners. It was reported that after the burner processing fibers become brittle. Furthermore, the temperature of an open-ended furnace is limited to less than 1300xc2x0 C., which requires tens of hours of diffusion time.
Resistive types of heaters, such as filament heaters, micro heaters and the induction heaters, have gained widespread acceptance in recent years for fiber processing. A distinct advantage of the resistive heaters is that the temperature can be controlled to 0.1%accuracy. However, the operating temperature is limited to less than 1700xc2x0 C. by the lifetime of the resistive heater itself, and such a temperature is not high enough for diffusing germanium in a short period of time. In addition, the thermal mass in the heaters also limits the temperature rise/fall time to more than 1 minute, which may unintentionally anneal and crystallize the fiber after the thermal processing. The relatively high price of the heater coupled with their short lifetime make them the least cost-effective compared with other alternatives.
CO2 lasers, by comparison, are free from these limitations. As the most widely used industry laser for more than two decades, CO2 laser is highly reliable and cost-effective. Typical lifetime of a CO2 laser tube is 35,000 hours or 17 years if running on a 40-hour work week basis. The laser wavelength, which is 10.6 xcexcm, is completely absorbed by silica and glass with an absorption length of about 10 xcexcm, making the laser beam a highly efficient heater. The xe2x80x9claser heaterxe2x80x9d has no thermal mass, and it is immune to the glass vapor deposition during the heating process. More importantly, the profile of the hot zone can be flexibly programmed simply by shaping or scanning the laser beam.
Despite these advantages, CO2 laser has not been able to be used for manufacturing fiber based components because of a simple fact: the laser power fluctuates by about xc2x15%. Heating appertain that utilize CO2 laser also utilize feed back loops that detect the laser power output and then change the amount of because of the small diffusion coefficient of germanium, in order to shorten the processing time the fiber temperature has to be raised to about 1800xc2x0 C. The attributes of commonly used heat sources are compared in Table I.
Traditional heat sources have been furnace and burners. It was reported that after the burner processing fibers become brittle. Furthermore, the temperature of an open-ended furnace is limited to less than 1300xc2x0 C., which requires tens of hours of diffusion time.
Resistive types of heaters, such as filament heaters, micro heaters and the induction heaters, have gained widespread acceptance in recent years for fiber processing. A distinct advantage of the resistive heaters is that the temperature can be controlled to 0.1% accuracy. However, the operating temperature is limited to less than 1700xc2x0 C. by the lifetime of the resistive heater itself, and such a temperature is not high enough for diffusing germanium in a short period of time. In addition, the thermal mass in the heaters also limits the temperature rise/fall time to more than 1 minute, which may unintentionally anneal and crystallize the fiber after the thermal processing. The relatively high price of the heater coupled with their short lifetime make them the least cost-effective compared with other alternatives.
CO2 lasers, by comparison, are free from these limitations. As the most widely used industry laser for more than two decades, CO2 laser is highly reliable and cost-effective. Typical lifetime of a CO2 laser tube is 35,000 hours or 17 years if running on a 40-hour work week basis. The laser wavelength, which is 10.6 xcexcm, is completely absorbed by silica and glass with an absorption length of about 10 xcexcm, making the laser beam a highly efficient heater. The xe2x80x9claser heaterxe2x80x9d has no thermal mass, and it is immune to the glass vapor deposition during the heating process. More importantly, the profile of the hot zone can be flexibly programmed simply by shaping or scanning the laser beam.
Despite these advantages, CO2 laser has not been able to be used for manufacturing fiber based components because of a simple fact: the laser power fluctuates by about xc2x15%. Heating appertain that utilize CO2 laser also utilize feed back loops that detect the laser power output and then change the amount of power to keep it to a constant level to within xc2x12.5%. This translates into more than 100xc2x0 C. uncertainty in temperature (for a temperature of about 2000xc2x0 C.), over which the fiber viscosity and diffusion rate usually change significantly.
According to an embodiment of the present invention a heating apparatus includes: (i) a laser providing at least one beam of light capable of heating a small area of an object; (ii) a laser driver adapted to adjust optical power of this beam of light; (iii) a photo-detector adapted to detect and measure thermal radiation from the small area; and (iv) a control loop operatively linked to the laser driver and the photo-detector, the control loop providing a signal to the laser driver to adjust optical power of the beam of light based on amount thermal radiation detected by the photo-detector. According to one embodiment of the present the laser is a CO2 laser and the small area is less than 0.25 mm in width. According to another embodiment it is a Nd: YAG laser. According to an embodiment of the present invention a method of heating a small area of an object includes the steps of: (i) utilizing a laser to provide a laser beam characterized by its optical power; (ii) directing the laser beam onto a small area with a cross-section of less than 1 mm; (iii) heating the small area with this laser beam; (iv) detecting thermal radiation radiated from the heated area; (v) adjusting, based on the amount of detected thermal radiation, the amount of the optical power.
It is an advantage of this invention that it improves the manufactrurability of the mode field expanded xe2x80x9csmartxe2x80x9d fiber tapers. A second advantage of this invention is that it provides a method of the splicing specialty fiber such as Er fibers, which are widely used in amplifier modules.
For a more complete understanding of the invention, its objects and advantages refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows.
It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.